Decompression surgery is a general term used to refer to any of a number of variations on the same basic procedure. The goal of the surgery is to create more space around the cerebellar tonsils and restore the normal flow of CSF. The procedure involves removing a piece of the skull in the back of the head near the bottom (craniectomy). Often part of the top one or two vertebra are also removed (laminectomy). At this point, depending on the individual case and doctor, some doctors will also open the covering of the brain, the dura, and sew a patch in to make it larger (duraplasty). There are many variations in how the surgery is performed, including (but not limited to) how much bone to remove, whether to open the dura, what type of material to use for a dural patch, whether to shrink or remove the cerebellar tonsils, and whether to replace the missing piece of skull with anything. Unfortunately, there is no consensus, and no strong evidence, on which technique(s) is the best. Because of this, it is important for patients to understand specifically what their surgeon will be doing and why. The procedure itself lasts several hours and most people will spend a night in the ICU and an additional couple of days in the hospital.
The primary problem in symptomatic Chiari malformation is crowding of the foramen magnum, the large opening at the base of the skull through which the spinal cord passes, by the descended cerebellar tonsils. To reduce this crowding and allow more room for the structures in this area as well as to improve flow of cerebrospinal fluid through the area, a Chiari decompression aims to create space at the level of the foramen magnum.
The procedure generally involves a linear incision at the back of the head and upper neck. A small part of the skull posterior to the foramen magnum is removed to effectively enlarge the opening, giving more room to the structures within. In many cases the posterior arch of the first cervical vertebrae is also opened to widen this space.
In most cases the dura, the leathery covering of the brain and spinal cord, is also opened in this area and expanded to improve the space within. This expansion, called a duraplasty, can be accomplished either with synthetic dura substitutes or sometimes with a piece of periosteum (the covering of the bone from the skull) from the patient.
Finally, in some cases in which the cerebellar tonsils are large, the tonsils may be removed or reduced in size to make further room for the other nervous system structures in the area.